I'm late to answer... I'm french, living in the US. I never heard of that in the US and nobody in France ever said something like that to me, but I never saw a OB there while pregnant... You might ask fr.misc.enfants.bebes they should know better about it. Nevertheless, I heard that vibrations can trigger miscarriage during the first trimester. I also heard that the miscarriage would have happened anyways because the featus is not "attached well"...
French have different views for pregnancy and baby care. For example, they don't like babies to ride in infant car seat for long. They call these car seats "cosy". They think it can cause back problems whereas in the US doctors think that you can even let babies sleep in them... I don't know if the French are too conservative or if the American are too attached to their car to put any limitation on driving...
Anyways stay informed in the practice of both sides and take what you think is best. I wouldn't change my plans, in the second trimester, the risk of miscarriage is very low and if she's driving a lot without any problem in France, I don't think that she will have problems while driving more in the US... But that's my opinion and I'm not a physicien. Maybe you can see OB in the US at the beginning of your trip for your peace of mind. Anne
